Faceplate means for improving dielectric strength of cathode-ray tubes

ABSTRACT

The invention is a multiple element faceplate assembly for cathode-ray tubes for increasing the dielectric strength of the faceplate and thereby eliminate voltage breakdown across the faceplate. A second faceplate is coupled to the fiber optic faceplate of a cathode-ray tube to compensate for flaws in the fiber optic faceplate which reduce the dielectric strength between surfaces of the fiber optic faceplate.

United States Patent Inventor Albert E. Oberg Horseheads, N.Y.

Dec. 10, 1968 June 15, 1971 Westinghouse Electric CorporationPittsburgh, Pa.

Appl. No. Filed Patented Assignee FACEPLATE MEANS FOR IMPROVINGDIELECTRIC STRENGTH OF CATHODE-RAY TUBES 3 Claims,4 Drawing Figs.

U.S. Cl 3l3/92LF, l78/7.85

Int. Cl H0lj 29/18 [50] Field ofSearch l78/7.85; 313/92 LF; 346/1 10[56] References Cited UNITED STATES PATENTS 3,043,179 7/l962 Dunnl78/7.85 X 3,043,910 7/1962 Hicks, Jr l78/7.85

Primary Examiner-Raymond F. Hossfeld AttorneysF. H. Henson, C. F. Renzand M. P. Lynch ABSTRACT: The invention is amultiple element faceplateassembly for cathode-ray tubes for increasing the dielectric strength ofthe faceplate and thereby eliminate voltage break down across thefaceplate. A second faceplate is coupled to the fiber optic faceplate ofa cathode-ray tube to compensate for flaws in the fiber optic faceplatewhich reduce the dielectric strength between surfaces of the fiber opticfaceplate.

PATENTEDJUHISIQH 3585432 50 FIG. 3. FIG. 2.

E Z FIG. 4 i l '9 21:7

WlTNESSES: INVENTOR W W/ 9 Albert EOb r erg Mf ATTORNEY FACEPLATE MEANSFOR IMPROVING DIELECTRIC STRENGTH OF CATHODE-RAY TUBES BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates in general tocathode ray tube faceplate structures and more particularly to means forimproving the dielectric strength thereof.

2. Description of the Prior Art In cathode-ray tubes, problems ofdeflection defocusing and deflection nonlinearity can be greatly reducedby use of a nonplanar phosphor screen whose center of curvature is thecenter of deflection of the electron beam. The use of fiber opticfaceplates has made possible the use of such a screen while stillproviding a flat viewing surface.

Furthermore the fiber optic faceplate; exhibits image transfercharacteristics far superior to that of ordinary glass panels andtherefore is used extensively in cathode-ray tubes for direct contactprinting of trace information on film.

The glass fibersof the fiber optic faceplate are of high indextransparent glass and are individually coated and separated from oneanother by a low index glass. Close bundling of the fibers in a parallelrelationship produces a faceplate which exhibits a high dielectricstrength between the faceplate surfaces. Due to the many thousands offibers present in the faceplate this ideal parallel relationship cannotbe uniformly maintained. A nonparallel relationship between adjacentfibers or an excessive separation between these fibers will result ineither the presence of voids in the surface of the faceplate or thepresence of an excess quantity of sealing glass, either of which reducesthe dielectric strength of the faceplate at this location.

In cathode-ray tubes wherein an electrically conductive coating isapplied to a phosphor screen which is in contact with the faceplate, avery high electrical gradient is established across the faceplate. Theelectrical gradient results from the application of a high electricalpotential, typically 16 kv., to the electrical coating and a zeroelectrical potential existing on the metal support structure in contactwith the exterior surface of the faceplate. In the application ofthefiber optic tube for direct contact printing of film, the film guide,which is maintained at a spacing of a few thousandths of an inch fromthe exterior faceplate surface represents an additional electricalconductor at zero electrical potential. The presence of such a highelectrical gradient across a fiber optic faceplate will likely result inelectrical breakdown at an area of low dielectric strength. Theelectrical breakdown results in potential puncture of the faceplate,damage to the tube phosphor and loss of vacuum. Furthermore the surge ofcurrent through the faceplate resulting from the voltage breakdownacross the faceplate presents a serious hazard to personnel in contactwith the faceplate.

The application of thin films of glass commonly used to seal fiber opticfaceplates does not provide the increased dielectric strength requiredto minimize high voltage electrical breakdown.

SUMMARY The invention is a cathode-ray tube employing a second faceplatemaintained in contact with the fiber optic faceplate of a cathode-raytube by suitable bonding or mechanical means to eliminate faceplatefailure due to high voltage electrical breakdown.

The addition of a second faceplate of either a fiber optic or glassconstruction, depending on the application of the cathode-ray tube,eliminates high voltage breakdown due to the improbability of areas oflow dielectric strength of the plates coinciding exactly.

The plate glass secondary plate is utilized in applications where a lenssystem is used to focus a beam on a remote target.

The fiber optic secondary faceplate is used in applications requiringdirect contact of the target with the faceplate such as direct contactrecording of trace information on film.

DESCRIPTION OF THE DRAWINGS FIG. 1. is a schematic view of a typicalembodiment of the invention;

FIG. 2 is a partial view in section of a single fiber optic faceplate;

FIG. 3 is a partial view in section of an embodiment of the invention;and

FIG. 4 is a partial view in section of an alternate embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a cathoderay tube 1 is shown comprising a n'eck portion 10 having a faceplate 11interconnected by cone 12. The faceplate 11 has a phosphor screen 13 inthe form of a material layer formed thereon which is excited uponimpingement of the electron beam 17 originating from the gun structure20. The gun structure 20 is located within the neck portion 10 and iscomprised of a cathode 21 and multiple grids 22 which control,accelerate, and focus electrons from the cathode 21 into the beam 17directed toward and convergent at the phosphor screen 13. Coils 28direct the electron beam 17 onto a prescribed spot on thephosphor'screen 13.

In accordance with standard practice in the fabrication of cathode raytubes a conductive coating 30 of a material such as aluminum is appliedto the phosphor screen surface remote from the faceplate 11 and isapplied continuously to the internal surface of the cone l2 and extendsinto the neck portion 10. The conductive coating in addition toimproving brightness and eliminating spot motion due to screen charging,provides an electrically field free space between the gun structure 20and the screen 13.

In operations of conventional cathode-ray tubes it is customary tomaintain the phosphor screen at a high positive DC voltage level incomparison to the cathode 21. For convenience it is customary tomaintain the cathode 21 and control grids 22 at or near ground potentialwith the screen 13 at a positive potential which may range from a fewkv. to 30 kv. or more. As illustrated in FIG. 1, the conductive coating30 is connected to a direct current high voltage source, typically 16kv., through the high voltage terminal 34 and the cable 36.

The cathode-ray tube is mounted in a suitable frame structure 40.

The structure and arrangement of components described thus far istypical for cathode-ray tube systems and is merely representative ofsuch systems.

The development of fiber optic technology has resulted in theapplication of fiber optics to tubes, such as cathode-ray tubes, in theform of improved faceplate structures.

The fiber optic faceplate may be formed by the use of a multiple fiberoptical image transfer assembly or bundle in which each of the many finefibers thereof are of high index transparent glass and are individuallycoated and separated from one another by a low index glass. This lowindex glass serves as optical insulation and insures good total internalreflecting characteristics insofar as the light traveling from one endof each fiber to the other is concerned. The nearly parallelrelationship of the individual coated light conducting fibers in thefiber optic faceplate produces an integral unitary optical imagetransfer from the surface of the faceplate 11 in contact with thephosphor screen 13 to the exterior surface of the faceplate 11. Thefiber optic faceplate therefore eliminates the diffusion of light andloss of contrast experienced in plate glass faceplates.

The fiber sizes are selected in accordance with the degree of resolutiondesired of the image which is to be transferred through the faceplate.Within practical limits smaller fibers in greater numbers provide higherdegrees of image resolution.

Due to the image transfer characteristics of the fiber optic faceplate,fiber optic faceplates have found considerable application in displaydevices for direct contact photographic printing of trace information.

An illustration of this application is depicted in FIG. 1 wherein thefilm 42 is brought in contact with faceplate 11 by the film guides 44.

While the fiber optic faceplate exhibits desirable opticalcharacteristics the structure of the fiber optic faceplate as used inconventional cathode-ray tubes cannot be depended upon to withstand thehigh potential gradient established between the conductive surface 30,which may range from a few kv. to 30 kv. or more, and the exteriorsurface of the tube which is at zero volts.

In the ideal fiber optic faceplate, wherein the individual fibers aresubstantially parallel to one another the dielectric strength exhibitedbetween the surfaces of the faceplate is high, typically millions ofvolts per centimeter and as such prevents voltage breakdown across thefiber plate due to the high potential gradient.

Generally, however, the alignment of the fibers is not uniformlyparallel and is more nearly represented by the fiber optic faceplateillustrated in FIG. 2. The faceplate 50 is comprised of fibers 55ofwhich fibers 55A and 55B represent nonparallel adjacent fibers. Thecrevice S created by the nonparallel relationship of the fibers 55A and558 represents a void in the faceplate surface or the presence of thelow dielectric insulating glass either of which reduces the effectivedielectric strength ofthe plate 50 at the location S. This reduction indielectric strength could result in a voltage breakdown occurringthrough the fiber optic faceplate 50 at location S between theconductive coating 30 and the mounting frame 40 or the film guides 44',the mounting frame 40 and the film guides 44 representing electricalconductors.

In FIG. 3 there is illustrated a detailed section of the faceplate ll ofthe cathode-ray tube of FIG. I. In addition to the fiber optic plate 14,the associated phosphor screen 13 and the conductive layer 30 there isprovided a second fiber optic faceplate 16 which is bonded to plate 14by a suitable optical cement 18. The second fiber optic plate 16, inaddition to maintaining the desired image transfer quality of fiberoptics, eliminates voltage breakdown across the faceplate structure 11by eliminating the adverse affects ofthe fiber misalignment illustratedin FIG. 2. The second fiber optic faceplate l6 acts as a seconddielectric element and provides adequate additional dielectric strengthat the low dielectric strength location S of plate 14 to prevent voltagebreakdown through faceplate 11. While it is acknowledged thatmisalignment of fibers in plate 16 can exist, the probability of thefiber optic plates 14 and 16 being positioned so as to align the lowdielectric strength locations of the respective plates is extremelyremote due to the thousands of fibers which are present in each fiberoptic plate.

The application of the second fiber optic plate 16 reduces therequirement for close control of the bundling of the fibers andtherefore improves the yield of acceptable fiber optic tubes.

While the second fiber plate I6 is shown as being bonded to the fiberplate 14, it is obvious that fiber plate 16 could be mechanicallysecured to fiber plate 14 by a suitable means (not shown). This methodof establishing intimate contact between plates 14 and 16 would permitreplacing the second fiber plate in the event of damage or excessivewear.

In those applications where direct contact with the fiber opticfaceplate is not required, a glass panel 19 of any desired opticalquality, transmission level, color or surface treatment can be bonded tofiber optic plate 14 to provide the same protection as that provided byfiber optic plate 16 as illustrated in FIG. 4.

Proper choice ofa means forjoining the second faceplate to the fiberoptic faceplate of the cathode-ray tube will permit the removal of thesecond faceplate and replacement thereof for maintenance reasons or toprovide the capability ofchangin the characteristics of the cathode-raytube faceplate.

arious modifications may be made within the spurt of the invention.

Iclaim:

1. In combination a cathode-ray tube display system having a faceplateassembly comprising,

a first faceplate of the fiber optic type, said faceplate-typeexhibiting random areas of low dielectric strength between the surfacesthereof,

a phosphor screen located on the interior surface of said firstfaceplate.

an electrically conductive coating on the phosphor screen remote fromthe interior surface of said first faceplate,

a second faceplate means for maintaining contact between a surface ofsaid second faceplate and the exterior surface ofsaid first faceplate toform said faceplate assembly, said faceplate assembly exhibiting greaterdielectric strength than that of said first faceplate, and

a second electrically conductive means associated with the surface ofsaid second faceplate remote from said first faceplate, and means formaintaining said electrically conductive coating at a high electricalpotential with respect to said second electrically conductive means toestablish a potential gradient therebetween, said faceplate assemblyexhibiting sufficiently high dielectric strength to substantiallyeliminate high voltage electrical breakdown between the remote surfacesof said faceplate assembly.

2. In combination as claimed in claim 1 wherein said second faceplate isof the fiber optic type.

3. In combination as claimed in claim 1 wherein said second faceplate isa glass panel,

1. In combination a cathode-ray tube display system having a faceplateassembly comprising, a first faceplate of the fiber optic type, saidfaceplate-type exhibiting random areas of low dielectric strengthbetween the surfaces thereof, a phosphor screen located on the interiorsurface of said first faceplate, an electrically conductive coating onthe phosphor screen remote from the interior surface of said firstfaceplate, a second faceplate means for maintaining contact between asurface of said second faceplate and the exterior surface of said firstfaceplate to form said faceplate assembly, said faceplate assemblyexhibiting greater dielectric strength than that of said firstfaceplate, and a second electrically conductive means associated withthe surface of said second faceplate remote from said first faceplate,and means for maintaining said electrically conductive coating at a highelectrical potential with respect to said second electrically conductivemeans to establish a potential gradient therebetween, said faceplateassembly exhibiting sufficiently high dielectric strength tosubstantially eliminate high voltage electrical breakdown between theremote surfaces of said faceplate assembly.
 2. In combination as claimedin claim 1 wherein said second faceplate is of the fiber optic type. 3.In combination as claimed in claim 1 wherein said second faceplate is aglass panel.